Control hoary alyssum in horse pastures

By Krishona Martinson and Roger Becker, University of Minnesota Extension

Due to last year’s drought and recent dry conditions in parts of the state, a number of people are struggling to control hoary alyssum. Hoary alyssum is a perennial weed common throughout Minnesota, the surrounding states and Canada. It is particularly adapted to dry conditions and sandy or gravelly soils. Hoary alyssum tends to increase in pastures and hay fields following drought or winterkill, regardless of the soil type.

Hoary Alyssum is toxic to horses when the fresh plant is grazed in a pasture, and when the dried plant is eaten in hay. Most hoary alyssum toxicity occurs when horses accidently ingest it in hay. Hay containing greater 20 percent or more hoary alyssum should not be fed to horses.

Horses ingesting hoary alyssum usually show clinical signs of edematous, swelling of the lower legs commonly called “stocking up.” Stocking up is usualy observed 12 to 24 hours after the horse ingests hoary alyssum. More severe clinical signs have been observed in horses ingesting hay with more than 20 percent hoary alyssum, including laminitis. Horses with laminitis may rarely have rotation of the coffin bone through the hoof, especially if transported during the acute phase of toxicosis. Transporting horses during this phase is not recommended. Clinical signs normally subside with supportive treatment two to four days following removal of the weed source; however, horses may not return to full performance fitness for a few months after onset of clinical signs.

If herbicides must be used in a grass pasture, the most effective and economical approach would be to apply 2,4-D in the spring and/or fall to suppress hoary alyssum. Retreatment will likely be necessary in following years. In a pure alfalfa hay field, herbicides applied to dormant stands or following first cutting can suppress hoary alyssum, but are expensive and can injure alfalfa. No herbicide options exist after first cutting. There are no herbicide options for controlling hoary alyssum in a grass/legume (alfalfa or clover) mixed pasture or hay field that do not have significant injury potential to the legume as well. When using herbicides, always read and follow the label carefully. Most herbicides have grazing and harvest restrictions that must be followed.

To minimize hoary alyssum and other weeds in pastures:
• Do not overstock or overgraze
• Have, and use if necessary, a sacrifice paddock
• Rotationally graze
• Remember that each pasture paddock needs two to six weeks of rest depending on the season (spring vs. summer vs. fall)
• Soil test every three years and fertilize accordingly
• Mow and drag after each rotation
• Control weeds (chemically, culturally and mechanically)
For more information on hoary alyssum and other toxic weeds, visit www.extension.umn.edu/horse/components/poisonousplants.htm.

(Krishona Martinson and Roger Becker are crops educators with University of Minnesota Extension.)